The present invention relates to a method of applying diffusion coatings by magnetron sputtering and in particular, to a method of applying multi-layer or multi-component diffusion coatings. The invention also relates to multi-layer diffusion-coated products.
A. Magnetron Sputtering
Magnetron sputtering is a well-known process for applying thin coatings onto objects. Sputtering is implemented by creating an electrical plasma over the surface of an target emitter material in a low-pressure gas atmosphere. Gas ions from the plasma are accelerated by electrical fields to bombard and thereby eject atoms from the surface of the emitter. These atoms travel through the gas environment until they impact the surface of the object to be coated, where they bond to the object, creating the coating layer.
A standard method of improving the efficiency of sputtering has been to use magnetic fields to confine electrons to the glow region in the vicinity of the emitter surface. The addition of such magnetic fields increases the rate of ionization which in turn increases the ion energy and the number of ions in the plasma. The increased ion energy and number of ions increases the overall sputtering rate.
Cylindrical magnetron sputtering devices are known which utilize elongated emitters and solenoid coils which produce flux lines parallel to the axis of the emitter. A significant drawback to such cylindrical sputtering devices is that they suffer from undesirable end effects. In a cylindrical magnetron, the direction of the electron drift velocity vector causes the electrons to orbit around the longitudinal axis of the emitter. However, the electrons tend to leak out or escape their orbits near each end of the emitter, resulting in lower ionization intensities and therefore lower sputtering rates at each end of the emitter. As a result, the portions of the object to be coated in the vicinity of the ends of the emitter may receive little or no coating.
Another drawback to cylindrical magnetrons is that in order to uniformly coat long objects such as pipes, a corresponding long vacuum chamber. emitter and solenoid coil must be provided, adding to the complexity and expense of the apparatus.
B. Diffusion Coatings
Diffusion coatings are coatings in which the coating material has diffused into the substrate such that atoms of the coating material and atoms of the substrate are intermixed. If the surface of the object is subjected to an elevated temperatures sputtered atoms will penetrate or diffuse through the surface of the object and intermix with the metallurgy of the object. It is well-known to create diffusion coatings using pack cementation techniques. As defined in ASM Metals Handbook, Volume 5, : xe2x80x9cDiffusion coatings are deposited either by heating the components to be treated in contact with the powder coating material in an inert atmosphere (solid-state diffusion) or by heating them in an atmosphere of a volatile compound of the coating material (out of contact gas phase deposition or chemical vapour deposition).
Diffusion coatings are known for their resistance to undesirable effects such as corrosion, oxidation, carburisation, erosion or hydrogen embrittlement, especially at elevated operating temperatures.
In Canadian Patent Application No. 2.175,439, filed Apr. 30, 1996, a diffusion coating, is disclosed which is referred to as a surface alloy. In this patent application, surface alloying may be achieved by a heat treatment subsequent to deposition of the coating. Alternatively, it is taught that surface alloying may be achieved during deposition by heating the substrate component to a temperature above 600xc2x0 C. In either case, it is clear that an external heat source is required to heat the component because no other method of heating the component is disclosed.
Creating a diffusion coating using magnetron sputtering requires that the object be externally heated. It is possible to heat the object to be coated by placing the object in a furnace to achieve the necessary temperatures which allows the creation of a diffusion coating. However, this usually requires placing the entire sputtering apparatus in the furnace which is cumbersome procedure.
C. Multi-Layer or Multi-Component Diffusion Coatings
It is often desirable to produce diffusion coatings with more than one component or layer of material, where each component or layer confers a certain performance characteristic to the coating. Aluminum-silicon codiffusion using pack cementation deposition techniques are well-known. However, simultaneous diffusion of other coating materials such as chromium, silicon and titanium have not been entirely successful despite being the subject of extensive study.
Although it is known to produce multi-layer diffusion coatings by sequential sputter depositing of individual layers, there has been no prior art disclosure of a method of depositing a multi-layer coating, in a single sputtering step.
Therefore, there is a need in the art for apparatuses and methods suitable for producing diffusion coatings using magnetic field enhanced magnetron sputtering. It would be further desirable to produce multi-component or multi-layer diffusion coatings with different properties through its cross-section.
In general terms, the invention comprises a method for applying a diffusion coating onto a workpiece using magnetic field enhanced magnetron sputtering, where the magnetic field is created by a solenoid coil which inductively heats the workpiece to a level where the coating diffuses into the workpiece. In a preferred embodiment, the workpiece is a tubular product such as a pipe or fitting which is made from stainless steel alloy.
Therefore, in one aspect of the invention, the invention comprises a method of diffusion coating a metal substrate by magnetic field enhanced magnetron sputtering wherein the substrate is inductively heated during coating deposition by a solenoid coil in a configuration of a coiled tubular conductor connected to a high current/high frequency AC power supply which produces the magnetic field. The substrate may typically be a metal pipe, tube or fitting and the solenoid coil may be external to and coaxial with the metal pipe, tube or fitting. Preferably, the AC power supply operates above a frequency of about 500 Hertz and more preferably in the range of about 1 kHz to about 2 kHz. The skin temperature of the substrate should reach at least about 350xc2x0 C. and should preferably be in the range of about 900xc2x0 C. to about 1200xc2x0 C.
The present invention also comprises a method of applying multi-component or multi-layer diffusion coatings. Therefore, in another aspect of the present invention, a method of diffusion coating a metal tube comprises the steps of:
(a) providing an emitter comprising at least two substantially concentric layers of differing composition;
(b) sputtering the emitter onto the metal tube such that the outermost layer of the emitter first diffuses into the surface of the metal tube and subsequent layers of the emitter diffuse into the coating surface thereby created; and
(c ) subjecting the metal tube to an elevated temperature during the sputtering step or in a subsequent heat treating step,.
Preferably, the outermost layer of the emitter comprises elements of relatively low solubility and the inner layer of the emitter comprises elements of relatively high solubility. The different layers of the emitter result in a layered diffusion coating on the metal tube wherein each layer confers different performance characteristics onto the metal tube.
In another aspect of the invention, the invention comprises a diffusion coated product which comprises a base alloy and a diffusion coating exterior to the base alloy wherein said diffusion coating comprises a plurality of coating layers each containing a mixture of a plurality of coating elements, wherein each coating layer has a composition different from the other coating layers. The coating elements may preferably be chosen from a group comprised of aluminium, silicon, titanium, chromium, zirconium, tungsten, vanadium, molybdenum, niobium, hafnium, tantalum and cobalt.
The elements may be chosen and applied such that the plurality of layers confer different properties to the diffusion coating, said properties chosen from the group consisting of: wear resistance, oxidation resistance, carburisation resistance, corrosion resistance or high temperature resistance.
Preferably, the diffusion coating comprises silicon, titanium and aluminum. The coating may comprise of three or more coating layers. Preferably, an outer layer is enriched in titanium, an intermediate layer is enriched in aluminum and an inner layer is enriched in silicon. Additionally a coating layer may be designed to act as a diffusion barrier or a thermal barrier.